Neutron Scattering - JUWEL - Forschungszentrum Jülich

6 J. Wuttke
NL6a
shutter
velocity selector
focussing guide
monochromator
Doppler drive
vacuum chamber
door
detectors
beamstop
chopper
sample
analysers
Fig. 4: Layout of the Jülich backscattering spectrometer SPHERES at FRM II.
Fig. 5: Schematic front view of the chopper rotor of SPHERES. The red bands indicate the
mosaic crystals that deflect the incident beam towards the monochromator.
these crystals, they undergo a Bragg reflection towards the monochromator. 2 Otherwise, they
are transmitted towards a beamstop.
The backscattering monochromator selects a neutron band Ei ± δE as described above. Neutrons
within this band are sent back towards the chopper. When they reach the chopper, the
rotor has turned by 60◦ : the mosic crystals have moved out of the way; the neutrons coming
from the monochromator are transmitted towards the sample.
The sample scatters neutrons into 4π. About 20% of this is covered by analyzers. If a scattered
neutron hits an analyzer and fullfills the backscattering Bragg condition, it is sent back towards
the sample. It traverses the sample3 and reaches a detector. To discriminate energy-selected
neutrons from neutrons that are directly scattered from the sample into a detector, the time of
arrival is put in relation to the chopper phase.
2 As a side effect, the Bragg deflection by rotating mosaic crystals achieves a favorable phase-space transform
(PST): the incoming wavevector distribution is spread in angle, but compressed in modulus. This results in a
higher spectral flux in the acceptance range of the monochromator.
3 Of course not all neutrons are transmitted: some are lost, some are scattered into a wrong detector. This inaccuracy
is inevitable in neutron backscattering. We strive to keep it small by using rather thin samples with typical
transmissions of 90% to 95%.